1. Field of the Invention
The invention relates to semiconductor wafer processing equipment and, more particularly, to a method and apparatus for tracking the location of defects on a substrate, e.g., a semiconductor wafer or a cleaved portion of a semiconductor wafer, by transforming a substrate coordinate system into a wafer analysis tool coordinate system.
2. Description of the Background Art
Identifying defects on the surface of a semiconductor wafer, such as particulate contaminants and other surface irregularities such as grain structure, is extremely important to integrated circuit manufacturing processes. To eliminate defect sources, defects are identified and analyzed to determine the source of the defect. Thereafter, corrective action can be taken to reduce or eliminate the defect source.
Typically, a defect identification process is accomplished in two steps. First, a laser scanner device scans a wafer with a laser and analyzes the backscatter from the wafer to locate defects on the surface of the wafer. One such laser scanner device is a Tencor SurfScan 6200, manufactured by Tencor Instruments. Secondly, the defect is analyzed to identify the root cause of the defect. The most commonly applied analysis tool is a high magnification imaging system such as a scanning electron microscope (SEM) which is used to identify the defect and/or the source of the defect by inspecting the defect at high magnification. Additionally, the SEM may be accompanied by instrumentation for performing chemical analysis of the defect. Such instrumentation includes an energy dispersive x-ray (EDX) detector. Other tools may include an Auger analysis device, atomic force microscope (AFM), optical spectroscopy device, electron backscattering detector and the like.
Many of these analysis tools can not physically accommodate an entire wafer within the analysis chamber of the tool. As such, the wafer is cleaved into manageable portions and each portion is analyzed separately within the instrumentation. One problem with cleaving a wafer into smaller individual portions and then attempting to analyze each portion is that the defect identified by the laser scanner device is identified in a coordinate system for the wafer as a whole as identified by the laser scanner device. Consequently, when the portion containing the defect is cleaved and placed within the analysis instrumentation, the coordinate system of the wafer that identifies the location of the defect is meaningless and the defect is not readily identifiable and positionable within the field of view of the analysis instrumentation. As such, the wafer analysis instrumentation must scan the entire portion of the wafer to find the defect for analysis.
A solution to this problem is disclosed in commonly assigned U.S. patent application Ser. No. 08/850,954 filed May 5, 1997 (Attorney Docket No. 1785) and incorporated herein by reference. This patent application discloses a method and apparatus for selectively marking defects on a semiconductor wafer. This apparatus fiducializes the wafer such that a two-dimensional wafer coordinate system is generated, then fiducializes the defects on the wafer identifying each defect and selectively marks each defect with an identifiable set of marks. These marks are readily visible via a two-dimensional, wafer analysis tool such that the defect can be easily located and analyzed. As such, when the wafer is cleaved into small portions, each portion containing a defect will have readily identifiable marks surrounding the defect.
Furthermore, the marks themselves can be used as a two-dimensional defect coordinate system for each defect that is contained within a cleaved portion of the wafer. As such, each wafer portion contains a two-dimensional coordinate system for the defect on that portion. The defect is known to be located a predefined distance from the marks. Consequently, a computer file can be generated for each portion of the wafer cleaved from an entire wafer that contains the two-dimensional coordinates of the defects on that portion relative to the defect marking coordinates (i.e., the defect coordinate system).
Although the method and apparatus for selectively marking a semiconductor wafer of U.S. patent application Ser. No. 08/850,954 filed May 5, 1997 (Docket 1785) provides an accurate, two-dimensional coordinate system for a defect and/or a cleaved portion of a semiconductor wafer, there are instances where the coordinate system requires transformation to conform to a particular analysis tool. For example, if the analysis tool is an electron backscattering diffraction (EBSD) analysis tool, the cleaved wafer is generally inserted into the analysis chamber at an angle relative to a horizontal plane. By analyzing the wafer portion tilted at an angle, the EBSD analysis tool has a small footprint within the laboratory and also promotes substantial electron backscattering due to the angle of incidence of the electron beam that scans the wafer portion. Other spectroscopic techniques such as Auger electron spectroscopy, optical spectroscopy and SEM may also tilt the wafer portion to achieve substantially improved signal strength and signal bandwidth. By tilting the wafer portion, the two-dimensional coordinate system associated with that portion is now distorted. Consequently, the defect is no longer directly identifiable by the two-dimensional coordinates of the wafer portion.
Furthermore, the analysis tool depth of field is limited and as such, the tool can only focus upon a small portion of the wafer. Thus, to find defect locations, not only must the tool scan in two dimensions, the tool must scan in a third dimension (focus) to maintain the surface within the depth of field of the tool.
Therefore, there is a need in the art for a method and apparatus that transforms the two-dimensional coordinate system associated with a tilted a wafer (or portion thereof) into a three-dimensional coordinate system of a wafer analysis tool.